____is also just fun___ people

fun后面 可以跟in doing sth其中in 可以省略
所以 答案是 A This ;watching我们是学过It's fun doing something，可这是this开头我认为 原句 与 It's fun doing something不一样这是 动名词作真主语it是形式主语 当然 原句 用it 更好 ，不过 用this 也不错不过句型不同而已。再如 i like singing english songs . this is fun (in) studying english.we had fun (in) playing with them.